Hydrocarbons, such as oil and gas, are commonly obtained from subterranean formations that may be located onshore or offshore. The development of subterranean operations and the processes involved in removing hydrocarbons from a subterranean formation are complex. Typically, subterranean operations involve a number of different steps such as, for example, drilling a wellbore at a desired well site, treating the wellbore to optimize production of hydrocarbons, and performing the necessary steps to produce and process the hydrocarbons from the subterranean formation. Different stages of a subterranean drilling and completion operation often involve data collection and transmission of data signals between different locations in the system.
For instance, in certain applications, it may be desirable to determine pressure at a location downhole. In certain implementations, a fiber optic-based pressure gauge device may be used to collect pressure data and relay that information to a desired location in the system. Operation of a pressure gauge device is often dependent upon downhole temperatures. Therefore, in order to obtain accurate pressure data, it may be necessary to also monitor changes in downhole temperature. Because temperature is a parameter of interest in its own right, the necessity for a second sensor to monitor temperature is not deemed an impediment and the gauges used are typically marketed as pressure/temperature point measurement gauges.
In certain applications, a pressure/temperature point measurement gauge (referred to herein as a “fiber gauge”) may include a pressure sensor and a temperature sensor. The temperature sensor of the fiber gauge may consist of a Fiber Bragg Grating (“FBG”) which can be placed in line with the pressure sensor. With this arrangement, a single fiber may be used to interrogate both the pressure sensor and the temperature sensor.
In certain implementations, the free response of the FBG may be obtained by attaching each side of the fiber to a support assembly having a clamp (or other suitable means), with the FBG section suspended. In order to avoid tension in the FBG, the fiber length between the clamps may be longer than the distance between the clamps, providing a certain degree of slack. The slack is provided to ensure that a change in temperature does not result in development of a tension in the FBG section of the fiber due to differential thermal expansion of the fiber and the support assembly. In certain implementations, the support assembly may be made of a metal having a coefficient of thermal expansion which is larger than that of the fiber which may be made of silica.
An alternate approach to measure temperature using an FBG is to couple the FBG to a metallic structure so that the FBG's response to a change in temperature includes the effect of the thermal response of the host material to which it is coupled. This implementation facilitates a higher sensitivity to changes in temperature due to the (typically) larger Coefficient of Thermal Expansion (“CTE”) of metals compared to silica. When using this approach, it is desirable to avoid elastic strain in the fiber host by decoupling the fiber host from outside forces to the highest degree possible.
In both the “free” and the “attached” implementations discussed above, space is needed in the assembly to accommodate the FBG and its supports. It is desirable to minimize this space in order to reduce the overall size of a pressure gauge incorporating such a device. Moreover, it is desirable that the FBG be exposed to the same temperature as the pressure sensor. Therefore, it is desirable for the FBG to be proximate to the pressure sensor and that thermal resistance between the FBG and the pressure sensor be minimized.
While embodiments of this disclosure have been depicted and described and are defined by reference to example embodiments, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.